It actually can't fail. It's a simple list of names associated with texture instances. addTexture is an extremely simple operation. If you can't assign a texture, chances are that it hasn't been added by your code. You can obtain all the names from the manager. Try to check if it contains the ones that you think it should.

It did because the loader and the texture weren't in the same directory. I fixed it by getting the absolute path. Would it be possible to add a line identifying the fact that the problem is coming from trying to load the texture, rather than when loading the model?